The reflectivity of electromagnetic radiation of long wavelengths, wavelengths longer than about 200 nm, on a conducting surface increases with the conductivity of the material of which the surface is made. It would therefore be expected that a superconducting surface would be essentially perfectly reflective. However, even given a superconducting surface, another limitation to perfect reflectivity is the band gap energy of the reflective surface material. If the band gap of the material is sufficiently low, the impinging electromagnetic radiation would at least in part be absorbed by the surface, rather than reflected. Thus, the low band gap of superconducting materials such as Nb.sub.3 Ge limits its use as a mirror to the microwave region. However, readily available nonsuperconducting metals also are extremely reflective of electromagnetic radiation in the microwave region, therefore the use of superconductive materials for reflectivity offered heretofore no substantial advantages.
Recently discovered superconducting materials which are superconducting above about 40.degree. K comprise multiple phases of oxides. See M. K. Wu, et al., Phy. Rev. Let., 58 (9), 908-910 (1987). The present invention is based in part on the discovery that the band gap energy for the multiple phase mixed oxide superconducting materials is greater than that of previously known superconducting materials. Applicants found that these new materials are useful for mirror surfaces for essentially perfect reflection of short wavelengths.
The present invention is further based in part on our recognition of the advantage that the thickness of superconducting mirror surfaces using the multiple phase mixed oxides may be very thin. The thickness of the superconducting mirror surface need only be sufficient to maintain its superconducting properties and surface integrity. This is an advantage over a non-superconducting mirror surface, the thickness of which is governed not only by structural properties required to maintain integrity of the surface, but also by the extent to which electromagnetic radiation penetrates a nonsuperconducting surface. For example, a superconducting metal oxide surface may be penetrated by light to a depth on the order of 100 Angstroms. Thus, very thin coatings of superconducting metal oxides might be used as mirror surfaces, with a thickness at least equal to the depth to which the reflected electromagnetic radiation penetrates the superconductor. Essentially a single thin layer may be placed on a substrate to form the superconducting mirror surface.